Question Line out and Bluetooth audio no longer work simultaneously

Nov 8, 2021
Hi. I'm having trouble using two headphones with my Windows 10 PC.
For years I've been using a Sennheiser RS 120 that uses FM technology. These cans are connected to the PC's line out.

I recently bought a Bose NC 700, cans that connect over Bluetooth. Added Bluetooth dongle to the PC and paired the NC 700. Worked fine. Sound was going both to the line out and the BT thingy, meaning I could switch between both headphones without having to adjust any settings.

As of today, this no longer works. I'm not aware of any changes I would have applied, but all of a sudden I have to choose between either line out or Bluetooth, selecting either 'loudspeakers' or the 'headphones' in the Sounds menu (Task bar -> Sounds -> Playback and then right-mouse click on either the loudspeakers or the headphones entry, to make them the default device). This works but is cumbersome.

To my surprise I'm googling articles that make it sound difficult to engage line out and BT simultaneously, yet this is exactly the scenario I had after pairing the Bose, without any further tweaks. I'm reading suggestions involving 'Stereo Mix' in the 'Recording' tab of the Sounds menu, but I don't see a Stereo Mix entry, not even when I tick 'Show disables devices'. I'm also reading this causes (further) audio degradation and (further) audio lag, which would be unwelcome. I'm also finding solutions that involve installing additional software, but this would mean starting up said software every day and going into its settings, all to do something that until yesterday happened automatically. So I would very much like to get back to the state of affairs as it was twenty-four hours ago!

Have looked into Device Manager, and Windows claims for all appropriate hardware that the best available drivers have been installed.

Any suggestions? I welcome any remarks!